Interleaving
Why Mixing It Up Beats Practicing One Thing at a Time
The Problem: Blocked Practice Feels Good But Fails
Traditional practice looks like this:
- 20 problems on addition
- Then 20 problems on subtraction
- Then 20 problems on multiplication
This is called "blocked" practice. One type at a time.
It feels effective. By problem #20, you're flying through them.
But it fails when it matters. On the cumulative test, students can't figure out which operation to use.
The Science: Interleaving vs. Blocking
Researcher Doug Rohrer ran a striking experiment with 4th graders learning prism volume problems.
| Practice Type | Description | Test Score (1 day later) |
|---|---|---|
| Blocked | All type A, then all type B | 38% |
| Interleaved | A, B, A, B, A, B... | 77% |
Same problems. Same total practice time. Double the results with interleaving.
Why Interleaving Works
The Discrimination Problem
When you practice one problem type at a time, you don't have to identify the problem type. You know it's multiplication because all the problems are multiplication.
But on a real test — or in real life — problems don't come labeled. You have to:
- Recognize what type of problem it is
- Select the right approach
- Execute the solution
Blocked practice only trains step 3. Interleaving trains all three.
The "Which Strategy?" Skill
| Blocked Practice | Interleaved Practice |
|---|---|
| "This is a multiplication problem. Do multiplication." | "What kind of problem is this? What approach fits?" |
| Strategy is given | Strategy must be chosen |
| Easy in practice, hard on tests | Hard in practice, easy on tests |
Why It Feels Wrong
Here's the frustrating part: interleaving feels less effective during practice.
| Metric | Blocked | Interleaved |
|---|---|---|
| Speed during practice | Faster | Slower |
| Confidence during practice | Higher | Lower |
| Long-term retention | Lower | Higher |
| Transfer to new problems | Lower | Higher |
Students (and teachers) often prefer blocked practice because it feels more productive. That feeling is misleading.
This is a classic "desirable difficulty" — harder practice produces better learning.
How ISP Applies This
Mixed Practice in TimeBack
ISP's curriculum doesn't segregate problem types into blocks. Instead:
| Traditional Math Homework | ISP Practice Session |
|---|---|
| Problems 1-10: Fractions | Problem 1: Fraction |
| Problems 11-20: Decimals | Problem 2: Area |
| Problems 21-30: Area | Problem 3: Decimal |
| Problem 4: Fraction | |
| Problem 5: Decimal |
Students must constantly identify the problem type before solving.
Spaced + Interleaved Review
Previously mastered material returns, mixed with current content. This combines:
- Spacing — seeing old material after a delay
- Interleaving — mixing old and new together
The double benefit: stronger memory and better discrimination.
Life Skills Curriculum
Even non-math content uses interleaving:
| Blocked Approach | ISP Approach |
|---|---|
| Week 1: All nutrition | Day 1: Nutrition + Mental prep |
| Week 2: All mental training | Day 2: Financial + Nutrition |
| Week 3: All financial | Day 3: Mental + Financial |
Students learn to recognize when each type of knowledge applies.
What This Looks Like in Practice
Monday, 8:30 AM:
Your son opens his math practice. The first five problems:
- Solve for x:
2x + 5 = 13 - Calculate the area of a rectangle: 4m × 7m
- Convert 0.75 to a fraction
- Solve for x:
x/3 = 9 - Find the perimeter of a triangle: 5cm, 7cm, 8cm
Each problem requires him to think: "What kind of problem is this? What approach do I use?"
It's harder than doing 5 equation problems in a row. That's the point.
Friday's test: He doesn't panic when problems are mixed. He's been practicing exactly this.
For Athletes: You Already Do This
Coaches don't run practice like this:
- 30 minutes of only free throws
- Then 30 minutes of only layups
- Then 30 minutes of only three-pointers
They mix it up:
- Drill 1: Free throw → sprint → layup
- Drill 2: Three-pointer → defense → free throw
- Scrimmage: All skills, random order
Why? Because games don't come in blocks. You need to read the situation and select the right response.
Academic learning works the same way.
When to Use Blocking vs. Interleaving
Interleaving isn't always better. Here's the nuance:
| Situation | Best Approach |
|---|---|
| First learning a brand new skill | Blocked — some initial massed practice helps |
| Practicing for retention and transfer | Interleaved — mixing builds discrimination |
| Similar concepts that are easily confused | Interleaved — forces noticing differences |
| Completely unrelated skills | Either — less benefit to interleaving |
ISP's system typically starts with short blocked practice for new concepts, then quickly moves to interleaved review.
The Research Behind This
| Researcher | Finding | Year |
|---|---|---|
| Rohrer | Interleaving helps students distinguish similar concepts | 2012 |
| Taylor & Rohrer | 4th graders: interleaved practice doubled test scores | 2010 |
| Kornell & Bjork | Interleaving helps learn painting styles (inductive learning) | 2008 |
| Dunlosky et al. | Interleaved practice rated "moderate utility" | 2013 |
FAQs
Q: Won't my kid get confused switching between topics?
A: That confusion is productive. It forces them to think about what they're doing, not just repeat a procedure mindlessly. Short-term confusion leads to long-term clarity.
Q: How do teachers grade interleaved practice?
A: The same as any practice. What matters is whether the student demonstrates understanding. Interleaving is about how practice is structured, not how it's evaluated.
Q: Does this work for all subjects?
A: Most effectively for subjects where students must distinguish between similar concepts or procedures — math, science, grammar, history analysis. Less critical for pure memorization tasks.
Related Pages
- Spaced Practice → — The timing dimension
- Desirable Difficulties → — Why hard practice is good
- Retrieval Practice → — The other half of effective practice
- Learning Science Overview → — All principles
"The goal isn't to get really fast at one thing. It's to get good at knowing which thing to do."